This invention relates generally to an analog electric timepiece and more particularly to an electric timepiece driven by a stepping motor having low power consumption and provided with a control circuit for preventing failure to accurately detect whether the stepping motor has rotated or not when driven. To improve the efficiency of electromechanical conversion in a stepping motor of low power consumption, such as an ultraminiature stepping motor for an electric wristwatch, a so-called correction drive method has previously been proposed. The correcting drive method is such that the rotor of the stepping motor is usually driven by a drive signal consisting of normal pulses of relatively narrow pulse width. However, if the rotor is not rotated by a complete step using the pulse of narrow width, for some reason or another, a correcting pulse of greater pulse width is applied. The most critical step in the correcting drive method is correct detection of a non-rotated condition of the rotor.
Various circuits have been devised to detect whether the rotor has in fact rotated when driven. When the rotor does not rotate a compensating pulse of sufficient pulse width is applied to assure that the rotor rotates and timekeeping does not lag. However, prior art circuits for detecting the rotated or non-rotated condition of the rotor have serious defects when the stepping motor is in an AC external magnetic field of low frequency such as 50 Hz or 60 HZ, or where there are high frequency fields such as occur, for example, when appliances such as electric blankets switch on and off. These high frequency and more conventional AC fields can affect the rotor windings and cause misreadings in the rotation detection circuit such that an indication of rotation is provided when in fact no rotation has occurred. In such a situation, the detection circuit will fail to provide the added pulse of greater pulse width which is necessary to keep the timepiece accurate.
What is needed is a stepping motor which advances accurately using driving pulses of narrow width for normal operation and longer pulses during heavy load operation. The stepping motor should operate reliably in the presence of AC magnetic fields and high frequency magnetic fields.